Hitherto, a remote keyless entry system has been developed which performs wireless communication via radio waves between a vehicle and a portable apparatus carried by a user of the vehicle to allow the doors of the vehicle to be locked or unlocked.
The remote keyless entry system includes: the portable apparatus that emits radio waves for instructing locking or unlocking of the doors by an operation of the user; and an on-vehicle apparatus that locks or unlocks the doors on the basis of the radio waves emitted from the portable apparatus.
In the remote keyless entry system which performs typical unidirectional communication, the on-vehicle apparatus is provided with an antenna device that receives the radio waves from the portable apparatus, and the portable apparatus is provided with an antenna device that emits radio waves for instructing locking or unlocking of the doors.
The antenna device of the portable apparatus is provided with a minute loop antenna obtained by forming a conductor into a loop shape. The antenna device of the portable apparatus supplies power to the minute loop antenna when emitting radio waves. It is known that at this time, an electric current flows through not only the minute loop antenna but also a circuit board on which the minute loop antenna is provided, so that radio waves are emitted also from the flowing electric current. That is, in the antenna device, radio waves are emitted also from the entire antenna device including the circuit board, in addition to the minute loop antenna which is intentionally provided by a designer. Thus, there is a problem that antenna performance cannot be obtained as intended by the designer.
Therefore, a technique has been developed in which the structure of the minute loop antenna is symmetrized as seen from a feeding point, thereby reducing the electric current flowing through the circuit board (e.g., Patent Document 1).